A time-of-flight (TOF) system, typically, includes a light source and a TOF sensor. The light source emits light pulses towards a target, which reflects the light pulses back towards the TOF sensor. The TOF sensor receives the light pulses after a time of flight, which is proportional to the distance from the TOF system to the target.
The TOF sensor includes a plurality of TOF sensor pixels. A conventional TOF sensor pixel 100 is illustrated in FIG. 1A, and a timing diagram for the TOF sensor pixel 100 is illustrated in FIG. 1B. The TOF sensor pixel 100 includes a photodetector 150, a first capacitor 160, a second capacitor 161, a first switch 170, and a second switch 171. An example of such a TOF sensor pixel is described in U.S. Pat. No. 7,683,954 to Ichikawa et al., issued on Mar. 23, 2010, which is incorporated herein by reference.
The photodetector 150 detects the light pulses emitted by the light source in response to a clock signal 101, as well as background light, to provide a photocurrent 102. The first switch 170 is controlled by a first control signal ΦA, which is, typically, the clock signal 101. The second switch 171 is controlled by a second control signal ΦĀ, which is, typically, an inverted clock signal (not shown). During the high portion of each clock cycle, the first switch 170 closes, connecting the photodetector 150 to the first capacitor 160. During the low portion of each clock cycle, the second switch 171 closes, connecting the photodetector 150 to the second capacitor 161.
Because of the time of flight, a first contribution to the photocurrent 102 from each light pulse is received by the first capacitor 160, and a second contribution from each light pulse is received by the second capacitor 161. A contribution to the photocurrent 102 from the background light is received by both the first capacitor 160 and the second capacitor 161.
The first capacitor 160 integrates the photocurrent 102 during the high portion of each clock cycle, over an integration period, to provide a first photocharge 103 of A+B, where A represents a first contribution from the light pulses, and B represents a contribution from the background light. The second capacitor 161 integrates the photocurrent 102 during the low portion of each clock cycle, over the integration period, to provide a second photocharge 104 of Ā+B, where Ā represents a second contribution from the light pulses. The difference 105 of A−Ā between the first photocharge 103 and the second photocharge 104 at the end of the integration period is related to the time of flight of the light pulses.
However, the contribution from the background light to the first photocharge 103 and the second photocharge 104 is, generally, much larger than the first and second contributions from the light pulses. Therefore, the first capacitor 160 and the second capacitor 161 may easily become saturated during the integration period. The large contribution from the background light also decreases the signal-to-noise ratio.
Furthermore, at the end of the integration period, two separate capacitor voltages, corresponding to the first photocharge 103 and the second photocharge 104, must be stored and processed. Moreover, to correct for distance aliasing, a second integration period is, generally, carried out, requiring that four separate capacitor voltages be stored and processed. These storage and processing requirements increase the system cost, in terms of memory, and the system latency.
Consequently, TOF sensor pixels have been developed in which a capacitor provides a differential photocharge, rather than separate first and second photocharges. In other words, these TOF sensor pixels perform in-pixel subtraction. Examples of such TOF sensor pixels are described in U.S. Pat. No. 6,919,549 to Bamji, et al., issued on Jul. 19, 2005, in U.S. Pat. No. 7,157,685 to Bamji, et al., issued on Jan. 2, 2007, in U.S. Pat. No. 7,176,438 to Bamji, et al., issued on Feb. 13, 2007, in U.S. Pat. No. 7,321,111 to Bamji, et al., issued on Jan. 22, 2008, in U.S. Pat. No. 7,683,954 to Ichikawa et al., issued on Mar. 23, 2010, and in U.S. Patent Application Publication No. 2011/0058153 to Van Nieuwenhove, et al., published on Mar. 10, 2011, which are incorporated herein by reference.
However, in these TOF sensor pixels, the in-pixel subtraction is only performed after a time period of several clock cycles. Therefore, measures must be taken to avoid capacitor saturation, adding to the complexity of the TOF sensor pixels. For example, a time period shorter than the integration period may be implemented by using a separate counter, or saturation threshold detection may be implemented by using a comparator. Furthermore, many of the TOF sensor pixels include multiple capacitors, and some further include multiple photodetectors. A simpler TOF sensor pixel that performs in-pixel subtraction is desirable.